Fluidized bed reactor having a furnace strip-air system and method for reducing heat content and increasing combustion efficiency of drained furnace solids

Abstract: A fluidized bed reactor (10) in which a bed of particulate material (25) is supported in a furnace section. A portion (25b) of the bed receives a greater amount of fluidizing gas to increase the stoichiometric conditions in the bed portion, strip relatively fine material from the particulate material in the bed portion and increase the amount of heat transferred to the flue gases. A cooler (40) is located adjacent the furnace section for receiving particulate material from the bed portion.

Description: This invention relates to a fluidized bed reactor and method for operating same and, more particularly, to a fluidized bed reactor utilizing a strip-air system for reducing the heat content of, and removing the relatively fine particulate material from, the waste solids drained from the furnace section of the reactor while at the same time increasing the reactor's combustion efficiency. Reactors, such as combustors, steam generators and the like, which utilize fluidized beds as their primary source of heat generation, are well known. In these arrangements, air is passed into the furnace section of the reactor and through a bed of particulate material contained therein which includes a mixture of a fossil fuel, such as coal, and an adsorbent, such as limestone, to adsorb the sulphur generated as a result of the combustion of the coal. The air fluidizes the bed and promotes the combustion of the fuel. To improve the pollution characteristics of fluidized bed reactors, it is known to stage the combustion of the fuel by controlling the amount of oxygen in various regions of the fluidized bed. In general, the lower region of the fluidized bed is operated under fuel rich or substoichiometric conditions such that nitrogen oxides emissions are reduced. The upper region is then operated under oxygen rich or oxidizing conditions to complete the combustion of the fuel. Each region of the fluidized bed is comprised of a homogenous mixture of particles of fuel and adsorbent, with a portion of the fuel particles being unburned, a portion being partially burned and a portion being completely burned; and a portion of the adsorbent being unreacted, a portion being partially reacted and a portion being completely reacted. The particulate material must be discharged from the system efficiently to accommodate the introduction of fresh fuel and adsorbent. To this end, a portion of the particulate material is usually passed from the lower region of the bed through a drain pipe to remove that portion from the reactor system. It has been found, however, that the particle size distribution in a fluidized bed, an important operating parameter, can be effectively controlled by recirculating part of this removed particulate material back to the furnace section. This is often accomplished by blowing air through the removed particulate material to strip away and entrain the finer portions of the particulate material and returning them to the furnace section. For example, in U.s. Patent No. 4,829,912 a method of controlling the particle size distribution in a fluidized bed reactor is disclosed in which the particulate material removed from the furnace section is passed through jets of air to entrain the finer portions of the removed particulate material by stripping them away from the larger solids and then recirculating these finer portions back to the furnace section. The non-stripped, nonrecirculated particulate material is passed to an ash handling system for removal from the reactor system. However, since this nonrecirculated particulate material has a temperature which exceeds the design temperature of common ash handling systems, the material must be cooled prior to its passage to the ash handling system. In these types of arrangements, the heat removed from the nonrecirculated particulate material can be put to productive use, such as to preheat combustion supporting gas or for reheat or superheat duty. A stripper/cooler located adjacent the furnace section of the reactor can both recirculate the finer portions of the removed particulate material and cool the removed but nonrecirculated particulate material. In these types of arrangements, a first, or stripper, section of the stripper/cooler receives the particulate material from the lower region of the fluidized bed through a drain pipe. Air is blown through the stripper section to strip, or entrain, some of the finer portions of the particulate material which portions are then returned to the furnace section. The particulate material remaining in the stripper/cooler is then usually passed to a second, or cooler, section of the stripper/cooler where heat is removed from the particulate material by passing water or steam in a heat exchange relation to the particulate material or by blowing air through it before it is discharged to the ash handling system. The stripper/cooler system just described is not without its drawbacks. For example, a significant portion of the particulate material removed from the furnace section of the reactor will be noncombusted fuel due to the usually substoichiometric conditions maintained in the lower region of the fluidized bed from which the particulate material is removed. This leads to less than optimal combustion efficiency for the reactor system since the removed noncombusted fuel is not recirculated to the fluidized bed due to its relatively large size. It is therefore discharged through the ash handling system. Further, as the particulate material is removed from the furnace section, it takes heat with it reducing the available heat in the furnace and requiring a cooling system to enable the ash handling system to manage the material. Moreover, duct work is required to return the stripped particulate material to the furnace section. GB-A-2189164 and FR-A-2 203 964 describes fluidized bed combustors where the plenum is divided into various sections and fluidising air is supplied to different parts of the plenum at different rates so that differing parts of the fluidized bed receive differing amounts of fluidizing air, the material removed from the bed being taken form that part which receives the greater air flow. It is therefore an object of the present invention to provide a fluidized bed reactor and method which has an improved combustion efficiency. According to the invention in one aspect there is provided a fluidized bed reactor comprising a furnace section, supporting means for supporting a bed of particulate material in the furnace section, plenum extending immediately below the supporting means, passing means for passing gas from the plenum through the supporting means and into bed, means for supplying gas at different gas flows to two sections of the bed selectively to fluidize the respective bed portions in the two sections, means for removing particulate material from the bed portion in the furnace section having the greater gas flow relative the other bed portion, and a vessel for receiving the removed particulate material, characterised in that partition means extend above the supporting means for partitioning that portion of the furnace section extending above the supporting means into the said two sections and means are provided to cool the removed particulate material in the vessel. Also according to the invention in another aspect there is provided a method for operating a fluidized bed reactor comprising supporting a bed of particulate material in furnace section above a plenum, passing gas from the plenum through supporting means and into the bed, supplying gas at different gas flows to two sections of the bed to selectively fluidize the respective bed portions in the sections, and removing particulate material from the bed portion in the furnace section having the greater gas flow than the other bed portion, characterised by partitioning the portion of the furnace section containing the bed of particulate material into two sections, and cooling the removed particulate material. By following the invention the heat content of the particulate material removed from the furnace section of the reactor can be reduced, the stoichiometry of a portion of the furnace section can be controlled independently from the rest of the furnace section, and the size of the stripper/cooler needed to receive particulate material from a fluidized bed reactor can be reduced. According to one embodiment of the present invention the air flow is increased by partitioning the plenum which fluidizes the bed and increasing the volume flow rate of fluidizing air passed into the draining portion of the bed. Alternatively, the air distributor nozzles which pass the fluidizing air from the plenum to the bed can be enlarged in the draining portion to decrease flow resistance and increase air flow. The invention will now be described, by way of example, with reference to accompanying drawing which is a sectional view of a fluidized bed reactor of the present invention. The fluidized bed reactor 10 of the present invention includes a generally rectangular furnace section 12 which is defined by front and rear walls 70 and 72 and side walls (not shown). A plenum floor 74 is provided at the base of the furnace section 68 and a roof (not shown) completes the enclosure. It is understood that if the reactor 10 is used for the purpose of steam generation, the walls would be formed by a plurality of heat exchange tubes formed in a parallel, gas tight manner to carry a fluid to be heated, such as water. It is also understood that a plurality of headers (not shown) would be disposed at both ends of each of the walls which, along with additional tubes and associated flow circuitry, would function to route the fluid through the reactor 10 and to and from a steam drum (not shown) in a conventional manner. These components are omitted in the drawings for the convenience of presentation. A perforated plate 78 extends horizontally in the lower portion of the furnace section for supporting a bed of particulate material 81. The bed 81 consists of discrete particles of fuel material, such as bituminous coal, which are introduced into the furnace section 12 by a feeder or the like in any known manner. It is understood that a sulphur adsorbing material, such as limestone, can also be introduced into the furnace section 12 in a similar manner which material adsorbs the sulphur generated by the burning fuel. It is also understood that a bed light-off burner (not shown) is mounted through the wall 70 above the plate 78 for initially igniting the bed 81 during start-up. A plenum 76 is defined between the plate 78 and the floor 74. The plenum 76 receives pressurized gas, such as air, from an external source via a conduit 80 under control of a damper 80a. Two sets of nozzles 82a and 82b extend through perforations provided in the plate 78 and are adapted to discharge air from the plenum 76 into the bed portions 81a and 81b. As shown the nozzles 82b fluidize the bed portion 81b and the nozzles 82a fluidize the bed portion 81a. The nozzles 82b have a larger cross-sectional area than the nozzles 82a and thus have a lower resistance to air flow than the nozzles 82a causing a higher volume flow rate of air to pass through them as compared to the nozzles 82a. Selective zonal fluidization of the bed portion 81b in relation to the rest of the bed 81, i.e. portion 81a, is thereby achieved by a passive system. A refractory lined enclosure 86 provided around the bed portion 81b to partition the bed portion 81b from the bed portion 81a. Suitable openings 86a and 86b are formed in the enclosure 86 to allow for the passage of particulate material and air between the bed portions 81a and 81b. The air passing through both of the bed portions 81a and 81b fluidizes the bed 81 to promote combustion of the fuel and combines with the products of combustion to form combustion flue gases which rise by convection in the furnace section 12. The flue gases entrain a portion of the relatively fine particulate material in the furnace section 12 and pass downstream to a separating section (not show) a heat recovery section (not shown). A cooler 40 is disposed adjacent the wall 72 of the furnace section 12, is generally rectangular in shape and is defined by front and rear walls 42 and 44 and side walls (not shown), a floor 50 and a roof 52. Whereas the walls are normally constructed of refractory lined plates, it is understood that if the reactor 10 is used for the purpose of steam generation, these walls could be formed by a plurality of heat exchange tubes in association with a plurality of headers and flow circuitry as previously described. A plate 54 is disposed in the lower portion of the cooler 40 and extends horizontally in the same plane as the plate 24 and spaced from the floor 50 to form a plenum 56 therebetween, it being understood that the plate 54 need not be disposed in the same plane as the plate 24. Two conduits 58 and 60 receive gas, such as air, from an external source and communicate with the plenum 56 at spaced locations to independently control the pressure in various portions of the plenum 56 as will be described. Dampers 58a and 60a are disposed in the conduits 58 and 60, respectively, to provide such independent control. A vertical partition 62 extends upwardly from the floor 50 to divide the plenum 56 into two sections 56a and 56b and to divide the cooler 40 into a cooler section 40a disposed above the plenum section 56a and a cooler section 40b disposed above the plenum section 56b. A passage is formed between the partition 62 and the wall 46 to allow particulate material in the cooler section 40a to pass to the cooler section 40b. The plate 54 is perforated and receives a plurality of nozzles 64 which are directed to discharge air from the plenum 56 to fluidize particulate material in the cooler sections 40a and 40b and direct the material from the cooler section 40a, through the passage 62a, to the cooler section 40b and toward a drain pipe (not shown) extending through an enlarged opening in the plate 54 and connecting with the cooler section 40b. A relatively large, generally horizontal duct 66 connects an opening formed in the wall 72 of the furnace section 81 to a corresponding opening formed in the adjacent wall 42 of the cooler 40 to permit the particulate material in the bed section 81b of the furnace section 12 to pass into the cooler section 40a of the cooler 40. In operation, particulate fuel material and adsorbent are introduced into the furnace section 12 and accumulate on the plate 24. Air from an external source passes into the plenum 76 via the air conduits 80, through the plate 24 and the nozzles 82a and 82b, and into the particulate material on the plate to fluidized the bed 81. The light-off burner (not shown) or the like is fired to ignite the particulate fuel material in the bed 81. When the temperature of the material in the bed 81 reaches a predetermined level, additional particulate material is continuously discharged onto the upper portion of the bed 81. The air promotes the combustion of the fuel and the velocity of the air is controlled by the damper 80a to exceed the minimum fluidizing velocity of the bed 81. The volume flow rate of the air introduced via the nozzles is also controlled to operate the lower region of the bed 81 under substoichiometric conditions to decrease the production of pollutants. To complete the combustion of the fuel, secondary air is supplied through air ports (not shown) into the upper region of the furnace section 12. As the fuel burns and the adsorbent particles are reacted, the continual influx of air through the nozzles 82a and 82b creates a homogenous fluidized bed 81 of particulate material including unburned fuel, partially-burned fuel, and completely-burned fuel along with unreacted adsorbent, partially-reacted adsorbent and completely-reacted adsorbent. Particulate material is drained from the bed portion 81b through the duct 66 to provide room for fresh fuel and adsorbent. The air flow into the bed portion 81b is maintained at a greater level than into the remainder of the fluidized bed 81a by reducing the flow resistance into the bed portion 81b. Therefore the volume flow rate of fluidizing air is increased which strips the relatively fine particulate material, increases the stoichiometric conditions, and cools the draining material. This increased air flow into the bed portion 81b strips the relatively fine particulate material from the draining solids, preventing these finer particles from entering the duct 66. The increased air flow also increases the percentage of oxygen in the bed portion 81b relative to the portion 81a which results in increased combustion of the fuel. A third effect of the increased air flow into the bed portion 81 is the increased transference of heat from the particulate material in the bed portion 81b to the flue gases. The damper 58a is opened as desired to introduce air into the cooler section 40a of the cooler section 40, via the plenum section 56a, to promote the flow of particulate material from the bed portion 81b to the cooler section 40 through the duct 66. The nozzles 64 are directed to discharge the air to urge the particulate material in the cooler section 40a and around the partition 62, which partition functions to increase the residence time of the particulate material in the cooler 40 before passing, via a drain pipe (not shown) communicating with the cooler section 40b, to the ash handling system (not shown). The velocity of the air and therefore the degree of flow of the particulate material into the cooler 40 and the degree of fluidization and cooling required are respectively controlled as needed by varying the position of the dampers 58a and 60a. The relatively cool air passing through the particulate material in the cooler 40 removes heat from the material and can be used as secondary combustion air in the furnace section 12 or in other ways, with proper openings and passages being added to the structure as needed. In addition, the heat resident in the particulate material in the cooler 40 can be transfered to a heat transfer fluid in either the walls of the cooler 40 or in a heat exchanger (not shown) disposed in the cooler 40. It is thus seen that the device and method of the present invention provides several advantages. The increased air flow strips away the relatively fine particulate material in the bed portion 81b and prevents it from draining. Therefore, the cooler 40 does not need a stripper section or the associated duct work needed to convey the stripped material back to the furnace section, thereby reducing the size and cost of the reactor system. In addition, the increased air flow cools the particulate material in the bed portion 81b by transferring its heat to the flue gases thereby reducing the amount of cooling required before passing the removed material to the ash handling system. The enclosure 86 provides the extra benefit of reducing the interaction between the bed portion 81b and the remainder of the fluidized bed 81. The plenum 76 can be partitioned to correspond with the bed portions 81a and 81b and provided with separately controlled air flows with the nozzles 82a and 82b then being identical. In this way the stoichiometry of the bed portion 81b drained from the furnace section 12 can be controlled independently form the rest of the furnace section. Thus, the air flow to the bed portion 81b can be increased to increase the stoichiometric conditions in that bed portion without affecting the substoichiometric conditions in the bed portion 81a. By increasing the stoichiometric conditions within the bed portion 25b, combustion is enhanced, resulting in less unburned fuel being removed from the furnace section 12. In this way there can be active control as opposed to the passive control with the different nozzles 82a and 82b. The duct 66 can be replaced by a generally vertical duct extending downwardly drom the bed portion 81b and the cooler disposed beneath the corresponding furnace section.

Claims: A fluidized bed reactor comprising a furnace section (12), supporting means (78) for supporting a bed (81) of particulate material in the furnace section (68), plenum (76) extending immediately below the supporting means (78), passing means for passing gas from the plenum (76) through the supporting means (78) and into bed (81), means (82a, 82b) for supplying gas at different gas flows to two sections (81a, 81b) of the bed selectively to fluidize the respective bed portions in the two sections (81a, 81b) and combust the fuel therein, means (66) for removing particulate material from the bed portion (81b) in the furnace section (12) having the greater gas flow relative the other bed portion (81a), and a vessel (40) for receiving the removed particulate material, whereby partition means (86) extend above the supporting means (78) for partitioning the furnace section (12) extending above the supporting means into the said two sections (81a, 81b) the partition means (86) including openings (86a, 86b) to allow the passage of particulate material between said two sections (81a, 81b) and means are provided to cool the removed particulate material in the vessel (40). The reactor of Claim 1 in which the means for supplying gas at different gas flows comprise two gas sources respectively connected to the two plenum portions, and two valves connected to the said sources. The reactor of Claim 1 in which nozzles (82a, 82b) extend from the plenum (26) through the supporting means (78) to supply gas to the bed, those nozzles (82b) delivering gas into the said other bed portion (81b) having a relatively larger cross-sectional area than the other nozzles (82a). The reactor of any preceding claim in which partition means (86) have openings (86a, 86b) to allow passage of the particulate material between the bed portions (81a, 81b). The reactor of any preceding claim in which the means (66) for removing particulate material comprise a duct (66) connecting the furnace section (12) to the vessel (40) for permitting the removed particulate material to pass from the furnace section (12) to the vessel (40). The reactor of any preceding claim further comprising a partition (62) in the vessel (40) for dividing the vessel into two portions (40a, 40b). The reactor of Claim 6 further comprising means for passing gas through the said vessel portions (40a, 40b) selectively to cool the removed material in those vessel portions. A method for operating a fluidized bed reactor comprising a bed (81) of particulate material in furnace section (12) above a plenum (76) the portion of the furnace section (68) containing the bed (81) of particulate material being divided into two sections (81a, 81b) by partition means (86), the partition means (86) including openings (86a, 86b) to allow the passage of particulate material between said two sections comprising the steps of: passing gas from the plenum (76) through supporting means (78) and into the bed (81), supplying the gas at different gas flows to two sections (81a, 81b) of the bed to selectively fluidize the respective bed portions in the sections and combust fuel therein under sub-stoichiometric conditions, and removing particulate material from the bed portion (81b) in the furnace section (12) having the greater gas flow than the other bed portion (81a), the removed particulate material being cooled. A method as claimed in Claim 8 in which the particulate material is removed to a vessel (40) divided into two portions (40a, 40b). A method as claimed in Claim 9 in which gas is passed through the vessel portions (40a, 40b) to cool the removed material.

Inventors: TOTH STEPHEN J, TOTH, STEPHEN J.
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1994
Main classification: F23C11
IPC codes: F23C10